1. Field of the Invention
The present invention generally relates an operational amplifier for image processing on a semiconductor circuit, and more specifically to an operational amplifier having selectable compensation feedback to insure stability for various modes of image processing.
2. Background Art
Conventional digital imaging devices, such as a digital camera or an optical mouse, utilize a photo-diode or an array of photo diodes to capture and record optical energy. The photo-diode converts optical energy to electrical energy (voltage or current) that can later be digitized and further processed.
The sensitivity of the photo-diode is limited by the “dark current” that is generated by the photo-diode. The dark current is the amount of current that is generated when no light is incident on the photo-diode, and it is desirable that the dark current be minimized or eliminated because it reduces the sensitivity of the imaging device. Dark current is especially a problem for digital camera and camcorder applications.
In order to minimize “dark current”, conventional imaging devices often utilize a specialized semiconductor process that is designed to minimize dark current in the junction of the photo-detector. For instance, the specialized process can be a CMOS process that has been optimized to minimize dark current. However, these specialized CMOS processes are often costly, and reduce yield. What is needed is an imaging device configuration that can be implemented using a conventional CMOS process, but that also minimizes dark current in the imaging device.
Prior to being digitized, the electrical energy from the photo diode array is often amplified with an analog amplifier. In consumer applications (such as cameras, etc.), the photo diode array has several operating modes such as reset, etc. Each of these operating modes causes different loading and bandwidth requirements on the analog amplifier. However, the analog amplifier must remain stable across these various operating modes. Accordingly, what is needed is analog amplifier that is configured to remain stable across the various operating modes of a digital imaging device, while also maximizing gain and bandwidth.
Further, over-exposure or saturation can occur when relatively bright sources of light are captured next to darker sources of light. When using multiple photo-diodes, it is possible for the output of one photo-diode (or a group of photo-diodes) to capture sufficiently bright light to saturate another photo-diode or group of photo-diodes, which degrades the overall image captured.
Conventional digital cameras utilize back-end software algorithms to address this saturation problem. However, these software tools tend to slow the operation of the digital camera due to the calibration period that is required.
Therefore, what is needed is a digital imaging device that can prevent over exposure at the hardware level to improve speed and bandwidth of the digital optical device.